请问“带类型常量”到底是怎么用的？

[syx007]

这是一段别人的程序，我把它复制到我的程序中，就出现：
[Error] PresideForm.pas(163): Left side cannot be assigned to
而在原来程序中没问题，我看不出有什么不同？
？“带类型常量”怎么用呢，帮一下忙！！
procedure TMainForm.Button1Click(Sender: TObject);
const
oldh : DWORD = 0;
begin
if not boolean(oldh) then
begin
oldh := GetWindowLong(Memo1.handle,GWL_WNDPROC);
SetWindowLong(Memo1.handle,GWL_WNDPROC,integer(@s));
end
else
begin
SetWindowLong(Memo1.handle,GWL_WNDPROC,oldh);
oldh := 0;
end;
end;

[mouseingrief]

up

[findcsdn]

这个const的作用和c的static的作用差不多属于静态变量。
{$j+} 方法可以，
也可以在project-->option-->compiler-->assignable typed constants 选中。

[chutian]

implementation

{$R *.dfm}
{$J+} //add this

注:
In the default {$J-} compiler state, typed constants can not have new values assigned to them; they are, in effect, read-only variables. However, if the {$J+} compiler directive is in effect, typed constants can have new values assigned to them; they behave essentially like initialized variables.